This invention relates to a blow molding method and more particularly to severing plastic between adjacent sections of a multi-station machine of the type operable in a vertical plane wherein the sections move radial to the axis during opening and closing.
In U.S. Pat. No. 3,785,761 and commonly assigned U.S. Pat. No. 409,652, filed Oct. 25, 1973, the inner sections of each of a plurality of closely adjacent blow molds are disclosed as resiliently mounted such that during traversal of an outwardly directed mold-opening cam portion by followers associated with the outer mold sections in the vicinity of 10 o'clock when looking at the rotary machine head-on, each inner mold section releases incrementally outwardly to cause plastic to break in the vicinity of the parting line of the next succeeding, still fully-closed mold. Such severing is advantageous because reliance for this on continuously sharp mold pinch lands is avoided, such lands tending to become dull after extended periods of use.
However, it has since been observed that when rotary molding machines of this general type are cycled at high rates, the glob of plastic in the tight area between closely adjacent mold sections (such closeness minimizing the amount of flash generated which requires reprocessing) is still quite pliable at the mold-opening portion of the cycle, due in large part to the fact such glob is not contacting any mold surface at all and thus is not being conductively cooled thereby. The result is that complete severance due to differential movement of adjacent molds is impeded, the material tending to only partially separate and to remain attached via strings on either side of the intended break point. As is also known, with this type of radial rotary molding machine the molded article is generally retained in the outer mold sections during mold opening by means of coaction between the plastic and a projection, such as an undercut, in the mold surface, thereby facilitating subsequent ejection from the machine at about the 12 o'clock position. With such incomplete severing as just described, any residual stringing of the material inhibits retaining the molded article in the outer section and frequently the strings prevent the undercut from functioning as intended, and instead of being in the outer section, the molded article is in the inner section at the point where it is to be ejected from the machine, which condition is not supportive of the accepted way of removing the molded products from the machine.